This Assassin Only Kills If Its Victim Strikes First

Letting your opponent land the first blow usually isn’t a surefire strategy for winning a fight, but for one insect predator, it’s the only way to come out on top.

As their name might suggest, assassin bugs are formidable killers. They’ve got a long proboscis that they use to stab prey, inject a tissue-liquefying toxic saliva and suck the pre-digested innards out for consumption. It’s like a harpoon, syringe, and straw all in one.

When brute strength and poison aren’t enough to make the kill or get out of trouble with their own predators, the bugs have a few other tricks. One species wears its victims’ flesh as a disguise. When it’s finished with its meals, the assassin tosses the drained carcasses onto its sticky back, building a cloak of corpses that disguises the bug and protects it from spiders.

Another species hunts spiders instead of hiding from them, and plucks the strands of their webs to mimic the vibrations of struggling prey. When the spider rushes over expecting a meal—Pow! Toxic proboscis sneak attack!—it becomes one instead.

Scientists have just discovered a different sort of tactic used by the species Ptilocnemus lemur, which hunts ants. It also deceives its quarry, but uses one of its own legs as a lure and lets an ant attack it before going in for the kill.

The assassin bug begins by waving one of its bristly hind legs around to attract nearby ants. Unlike all the other lurer hunters that the researchers know of, the assassin doesn’t strike when an ant gets within range, or even when it touches or grasps the bait. It just waits.

Finally, when—and only when—the ant firmly grips the bug’s leg and either attempts to sting it or drag it away, the assassin leaps into action. With the ant still hanging on to its leg, P. lemur lifts itself up and swivels 180 degrees on its “knee” joint. Now its body is above the ant, and perfectly positioned to attack a weak point in the prey’s hardened exoskeleton—a soft membrane between the head and thorax.

Proboscis goes in. Nutritious goo comes out.

As the researchers, led by Australian ecologist Matthew Bulbert, watched, the assassins successfully killed 81 percent of the ants that fell for their trick. In almost 500 of these attacks, though, not one assassin bug died.

Ants are dangerous game and adept insect killers themselves. Some species are also several times bigger than the assassin bugs. Using its own leg as bait with them would seem to put the assassin in harm’s way, but Bulbert and his team think the seemingly risky strategy of letting the ant bite first actually reduces the bug's chances of getting hurt.

By presenting its leg and waiting until the ant latches on, the assassin has tricked the ant into concentrating all its offensive traits—its size, its jaws and its venom—onto one body part. Once the ant attempts to claim its meal, its entire arsenal is occupied, and it's unable to defend itself from an quick counter-attack that comes from above and behind its head. By taking a blow to the leg, the assassin bug turns the hunter into the hunted “unopposed and essentially risk free,” the researchers say.

Crafty and effective as it is, the strategy also has some constraints. Only 2.5 percent of the ants that the researchers saw actually grabbed the assassin bugs’ legs. But the slim chance of an ant falling for the trick, Bulbert thinks, is offset by their size. The average ant that the assassins preyed on was one to two times larger than its killers, and could feasibly provide enough food to keep an assassin going through some lean times. In some cases, the ants were as much as five times bigger than the assassin, and the bug lost all contact with the ground when it flipped over on top of its prey and had to ride on the ant’s back until it died.